Potatoes are a pretty easy vegetable crop to grow, but they aren’t
the easiest to grow using organic methods. That’s because they are prone
to quite a few insect pests and diseases that can be a challenge to manage
organically. They also require relatively high levels of available nutrients
in order to obtain good yields.
A recent study by Cornell University (Farm Ecosystem and Management
Factors Contributing to Pest Suppression on Organic and Conventional Vegetable
Farms) compared several wholesale potato farms in New York. Yields on organic
farms were found to be lower than yields on conventional farms, in large
part because of damage by potato leafhopper, but profits were higher because
organic potato prices were higher than conventional prices. Even with a
price premium, good organic growing practices are needed in order to assure
decent marketable yields.
It Starts with Seed. Organically grown ‘seed’ potatoes
are considerably more expensive than their conventional cousins. The national
organic standards require the use of organic seed (and vegetative propagules
like tubers) unless they are ‘commercially unavailable’
Consult your certifying agency as to how they are interpreting that
phrase, since the USDA has not yet defined it. At the very least, you will
need to show that you made a good faith effort to locate the type and quantity
of organic seed that you need to plant.
Variety Selection. Choosing varieties is very important for
marketing, storage and pest management reasons. Obviously, you need to
grow what your customers like - if you can’t sell the crop then it hardly
matters if was able to tolerate diseases or last through the winter.
There are so many potato varieties to choose from. A few that are
common in Northeastern organic markets are Chieftain, Norland, Superior,
Yukon Gold, Carola, Yellow Finn, Russian Banana, All Blue, and Caribe,
but many other varieties are grown.
Weighing the pros and cons of a variety can get complicated. For
example, according to the University of Idaho, Norland is an early round
red variety that produces good yields, with moderate resistance to common
scab but it’s relatively susceptible to Verticillium and tends to loose
color and weight in storage. Russet Burbank has good storage, processing,
and culinary qualities but it’s susceptible to environmental stresses leading
to tuber defects, and it may not yield as well in the east. Yukon Gold
has a high level of consumer acceptance and name recognition but it’s susceptible
to internal tuber defects and common scab.
Jim and Meg Gerritsen grow organic seed potatoes at Wood Prairie
Farm in northern Maine. For growers with insect problems they recommend
Prince Hairy, a variety that has tiny hairs on its leaves. That helps it
resist Colorado potato beetle (CPB), potato leafhopper (PLH) and flea beetles.
Some growers do not like the taste of Prince Hairy, and it is also susceptible
to scab. Maybe Elba is a variety to try since it has high scab resistance,
tolerance to tuber late blight, and seems to tolerate PLH relatively well.
Soil management. As with most crops, adding organic
matter to the field can improve soil quality for potato production. However,
with potatoes, if the organic matter is too fresh it may lead to soil borne
diseases like scab and Rhizoctonia. And while it’s common knowledge that
lowering the soil pH can help avoid problems with scab, most organic producers
grow potatoes as part of a diversified vegetable rotation, so lowering
the soil pH is not a viable way to control potato scab since it negatively
impacts the yield of other vegetable crops.
To avoid diseases such as Verticillium and insects such as potato
beetle, fields should be rotated out of solanaceous family crops (tomatoes,
peppers, eggplant, tobacco) for at least 3 years. To minimize potato beetle
pressure, it is ideal to plant potatoes a long distance from last year’s
fields, and if possible to have a road or river between the two.
Legume cover crops should not be grown ahead of potatoes, since this
can encourage scab, nor should sod crops, since they may increase wireworm
populations. On the other hand, small grains, corn or sorghum-Sudangrass
may benefit a potato crop that follows. In Maine, some growers have used
Japanese millet as a cover crop in the year prior to potatoes in an effort
to reduce Rhizoctonia, which causes symptoms on potato skin that looks
like ‘dirt that won’t wash off.’
Many organic growers do not apply compost directly before planting
potatoes, in order to keep scab pressure down. Instead, potatoes occupy
a place in the rotation where they can use residual nitrogen from heavier
applications of organic matter amendments in previous years.
Potatoes produce a lot of biomass, so it makes sense that they have
relatively high N, P and K requirements. To meet soil test recommendations,
organic growers may need to apply more bagged nutrients to potatoes that
to most other vegetable crops. Placing these materials in the row prior
to planting in one way to minimize cost and avoid feeding weeds between
Pest Management. Mechanical weed control is relatively
easy in potatoes, because they are hilled. But don’t delay; cultivate shortly
after the plants emerge, then hill frequently, at no more than 2-week intervals,
until the canopy closes. This usually keeps weeds from getting a foothold.
CPB and PLH are the two main insect pests for most organic producers.
One you can see easily, and the other one may take you by surprise. CPB
overwinters adjacent to potato fields and the emerging adults are hard
to miss, hanging out on the plants and laying eggs that soon hatch into
slug-like larvae. Both adults and larvae can be killed with an organic
formulation of Spinosad. Small larvae can be selectively controlled with
a product containing the B.t. bacterium that is toxic only to CPB and their
close relations. Unfortunately, there are currently no organically-approved
formulations of B.t. for use on Colorado potato beetles.
Potato leafhopper does not overwinter in most locations but arrives
on the wind. The damage they cause is called hopperburn, and it makes the
foliage to go down early in the season, reducing yields. Because PLH are
somewhat hard to see due their small size and light green color, they are
often not detected by growers until populations are quite high and a ‘cloud’
of them appears when plants are brushed. Scouting leaf undersides and axils
is the best way to note their arrival and have time to treat before they
build up. Organic insecticides are not particularly effective, but
growers have had some success with combinations of neem extract and pyrethrins.
Good coverage of foliage is crucial.
Potatoes are subject to many diseases and physiological disorders.
Early and late blight, potato scab, rhizoctonia, and hollow heart all commonly
affect organic as well as conventional producers. Timing can also help,
for example, the threat of late blight can be minimized by growing early
varieties that mature before windborne late blight spores become common.
Prompt harvest (along with use of disease-free seed) can minimize infection
by Rhizoctonia because a lot of infection occurs after the vines die. Hollow
heart, a disorder caused by too-rapid growth, can be minimized by close
spacing and relatively early harvest.